This invention pertains to improvements in apparatus for performing dental x-ray examinations. The invention features a better way to shield a dental x-ray tube head for preventing emanation of unwanted stray radiation, while still retaining effective insulation against the high voltages produced within the x-ray tube.
Dental x-ray apparatus customarily has the x-ray tube enclosed in an oil-filled housing called a tube head. One face of the tube head has an opening through which the primary x-ray beam is projected from the x-ray tube target toward the examination subject. A tubular member, called a cone, is coupled to the tube head axially of the opening to assist in aiming the x-ray beam at the proper zone on the subject""s face. Early in this art, the x-ray tube head or housing had been completely lined with an x-ray shield such as sheet lead to prevent the stray radiation, that is, radiation other than that in the primary beam, from emanating in all directions through the walls of the housing. Most of the sheet lead is usually disposed on the inside wall of the housing so that it encloses a volume which is substantially the same as the volume of the housing. One reason for arranging the lead sheet in this way is to avoid having it be too close to the x-ray tube and other components in the tube head which have high voltage, in the range of 70 to 100 KV, applied to them.
The use of lead for shielding results in a tube head that has undesirably great weight and size. A consequence is that the pantograph type of tube head support arm, which is usually used to support the tube head for movement with all degrees of freedom, must be designed for handling the unduly high weight. It is more difficult to support a heavy tube head in such manner that it will remain in equilibrium when positioned near the patient than it is to support a lighter tube head.
To date, the conventional solution to the problem of excessive weight of lead shielding is partially exemplified in O""Connor, U.S. Pat. No. 4,157,476. O""Connor shows a dental x-ray tube head wherein the x-ray tube is partially enclosed by a member formed of a resin impregnated with barium sulfate. The resin shielding member, however, is open in several areas. Besides the side area from which the intended x-ray beam is to be emitted, the resin shielding member is also open at the end, to permit the connection of wires. It was found in putting the O""Connor arrangement to actual use that the insulating oil had insufficient x-ray attenuation properties to be relied upon to stop the x-radiation from escaping from the open end of the resin shielding member.
Various solutions have been attempted to resolve this latter problem. One solution was to manufacture the outer casing of zinc, rather than the aluminum admitted by O""Connor to have poor x-ray attenuating properties. Zinc, however, has a severe weight disadvantage when compared to aluminum. Another solution attempted was to manually form a lead shield over the open end of the resin shielding member. As indicated in O""Connor, though, lead is toxic and is a health hazard to persons involved in making the shield. Moreover, when pieces of lead sheet are used for shielding, there is a greater chance for radiation to leak through a joint. Even further, lead shielding is an electrical conductor, requiring the addition of electrical insulation, or at least an increase of spacing, surrounding the lead shielding. Lead shielding can also cause air bubbles to form in the insulating oil, again reducing the electrical insulation and radiation shielding.
This invention relates to improvements to the apparatus described above, and solutions to the problems raised and/or not solved thereby.
In accordance with the invention, in a dental x-ray tube head comprising a housing having an x-ray tube mounted in it, there is improved shielding for preventing stray radiation from emanating outside of the path of the primary x-ray beam. The shielding includes an inner hollow element encasing the x-ray tube and having an aperture through which the primary beam projects, with an open end for connecting wires to the x-ray tube. The shielding further includes an outer hollow element which fits over the inner element in such a way as to cover the open end, with clearance for the wires connected to the tube. Both the inner element and the outer element are comprised of a mixture of polypropylene and barium sulfate. The combination of the inner element and the outer element completely surrounds the x-ray tube with the barium sulfate impregnated material, eliminating the use of lead within the tube head, and thereby providing excellent electrical insulation characteristics besides the x-ray attenuation. This combination also permits the tube head casing to be formed of aluminum rather than zinc, with the all the weight advantages attendant thereto.